Please post an excerpt from your Google calendar export which contain such an event. All events in your Google calendar file, which you’ve imported into the Nextcloud database, should contain valid time zone information.
I’ve just found the following thread which seems to describes your problem.
The problem occurs on at least three different PCs and I tested it with Firefox, Chrome and Edge. So I guess this is not the solution in my case, because I’m sure this would mean that everyone has this problem.
And why are the times correct in the overview, but not in the detail view?
I analyzed the Google export files again and found out that 3 of 7 calendars have this at the top (below of what I already posted):
I had the exact same problem. After investigating it for quite some time, tinkering with the ics file manually etc. I found out that Nextcloud just correctly imports what’s in the ics file exported from Google. So the issue here is Google exporting calendar events partly with datetimes in UTC (for me almost all of them) and partly in the local timezone.
The only way to fix it and to get to the desired outcome (having all calendar events in local time and with local timezone) is writing a script which converts datetimes to local time and also sets the correct timezone for all of the datetimes in the ics file. Very similar to the Python example linked in a post above.
I put my (very quick and dirty) script written in Ruby in this Gist.
Thanks for your reply. I still had no time to investigate the problem further, but I have it on my to-do list. A script for rewriting the ics files from Google comes too late for me, unfortunately. I use the NC calendar for too long now and have too much new entries. So I think I have to edit the events in the database of Nextcloud.